Follow steps below to enter the DFU mode and scan your iPhone 3GS for lost contacts.

Step 1: Hold your iPhone 3GS and click “Start” button.
Step 2: Hold “Power” and “Home” buttons on your iPhone 3GS at the same time for exact 10 seconds. Dr.Fone(iPhone 3GS) will count time for you.
Step 3: When 10 seconds passed, release the “Power” button right away, but keep holding the “Home” button. After another 10 seconds, you’ll be informed that you’ve successfully entered the DFU mode and you can release the “Home” button now.

Then Dr.Fone(iPhone 3GS) begins scanning your iPhone automatically for lost data, and you’ll get the interface as follow.

If you only want to recover deleted contacts, you can refine the scan result by sliding the button in the red area on the interface above, only displaying the deleted contacts. Mark them and press "Recover" button to get them back.